Peters’ office issued a statement — without mentioning his role as treasurer — saying that to avoid the appearance of conflict of interest, he would step away from DOI’s probe of the matter.

“As has been reported, the Department of Investigation, in conjunction with its law enforcement partners, is investigating allegations related to certain (de Blasio) campaign donors,” the statement read. “In order to avoid the appearance of a conflict, the commissioner has recused himself from these matters.”

Read additional:

The Mark Peters principle: The city’s investigations chief, former de Blasio campaign treasurer, won’t recuse himself from a fundraising investigation. Bad move.

“Peters draws a bright line invisible to the naked eye. Any possibility of improper influence colors the entirety of this donor’s giving — and any investigation must go from the very start of a timeline that begins, in de Blasio’s own telling, with his victory in the September 2013 primary.

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Presciently, Council members Jumaane Williams and Daniel Garodnick fretted about this very scenario in Peters’ January 2014 confirmation hearing.

“I can’t think of an instance where . . . I would need to recuse myself from a matter involving the mayor,” the nominee harrumphed.

Are you sure? asked Garodnick. What if the Campaign Finance Board came calling about the de Blasio campaign?

“I hadn’t thought of that,” Peters admitted, and then restated: “If the Campaign Finance Board were to make a referral to DOI involving the mayor’s 2013 mayoral campaign then yes, for that I would recuse myself.”

Oh, nothing like that to see here — just a relentless, corruption-busting U.S. attorney chasing some very bad behavior closer and closer to de Blasio’s doorstep.”

Some of the police officials interviewed responded later to follow-up questions from the agents, who are assigned to a squad that investigates public corruption, one of those briefed on the investigation said. The inquiry is being conducted with prosecutors from the United States attorney’s office in Manhattan, and a federal grand jury has begun hearing evidence in the case, two of the people familiar with the matter said.

The two businessmen, Jona Rechnitz and Jeremy Reichberg, served on Mr. de Blasio’s inaugural committee in 2013, along with more than 70 others, including the actress Sarah Jessica Parker, the author Junot Díaz, the billionaire businessman Ronald O. Perelman, the real estate developer Steve Witkoff and Mark G. Peters, who was Mr. de Blasio’s campaign treasurer and is now the commissioner of the Investigation Department.

The precise allegations under investigation are unclear; neither man has been charged with a crime.